Reconsider the buffet. It's a common myth that buffet or station setups are always more budget-friendly. In reality, they may require extra staff (to refresh the food), china and flatware, which drives up costs.
Have guests order in advance. Including an entree selection line or insert with your invites instead of offering a choice at the table allows your caterer to plan ahead and purchase only the necessary quantities.
Choose local foods. Opting for seasonal fare that's locally harvested can be cost-effective. Stay away from items that have to be shipped in (steak will be less expensive than seafood if you're getting married in a landlocked locale, while the opposite's true of the coast).
Stick to small quantities. If you're set on serving a fancy food like lobster, offer it over a bouquet of baby greens as the appetizer instead of the entree. It still looks posh, but you'll pay a lower price.
Find out everything you need to know about planning your menu here.